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SATELLITE NETWORKS

2000. 11. 27

이 은 진

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Contents

• Introduction

• Polling

• ALOHA

• FDM(Frequency-Division Multiplex)

• TDM(Time-Division Multiplex)

• CDMA(Code Division Multiple Access)

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Introduction

• Communication satellites have up to a dozen or so transponders.

• Each transponder has a beam that covers some portion of the earth.

• Stations within the beam area can send frames to the satellite on the uplink frequency.

The satellite then rebroadcasts them on the downlink frequency. Different frequencies are used for uplink and downlink to keep the tra

nsponder from going into oscillation

• Problems that occur with satellite-based wide area networks. - design issues is how to allocate the transponder channels. - the downlink channel has only a single sender(the satellite) and thus has no channel allocation problem. - The main problem is with the uplink channel allocation problem.

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Polling Having the satellite poll each station in turn to see if it has a frame is prohibitively expensive, given the 270-msec time required for each poll/response sequence. If all the ground stations are tied to a packet-switching network - to arrange all the stations in a logical ring, - each station knows its successor. - around this terrestrial ring circulates a token. - The satellite never sees the token. - A station is allowed to transmit on the uplink only when it has captured the token.

토큰

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ALOHA Pure ALOHA - Every station just sends whenever it wants to. - The trouble is that the channel efficiency is only about 18 percent.

Slotted ALOHA

- Using slotted ALOHA doubles the efficiency but introduces the problem of how to synchronize all the stations so they all know when each time slot begins.

- One ground station, the reference station, periodically transmits a special signal whose rebroadcast is used by all the ground stations as the time origin.

- To increase the utilization of the uplink channel above 1/e, go from the single uplink channel to the dual uplink scheme.

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ALOHA

- If one of the uplink channels contains a single frame, it is just transmitted in the corresponding downlink slot later.

- If both channels are successful, the satellite can buffer on of the frames and transmit it during an idle slot later on.

- The downlink utilization can be gotten up to 0.736 at a cost of increasing the bandwidth requirements by one half.

Continued

Satellite Satellite

Downlinkchannel

Uplinkchannel

Onedownlin

kchannel

Twouplink

channels

< 표준 ALOHA 시스템 > < 두 번째 업링크 채널의 부가 >

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FDM(Frequency Division Multiplex)

The oldest and probably still most widely used channel allocation scheme.

A TYPICAL 36-Mbps transponder might be divided statically into 500 or so 64000bps PCM channels,

each one operating at its own unique frequency to avoid interfering with the others.

Disadvantages

• Guard bands are needed between the channels to keep the stations separated.

• The stations must be carefully power controlled.• FDM is entirely an analog technique and does not lend

itself well to implementation in software.

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SPADE SYSTEM (single channel per carrier, pulse code modulation, multiple-access, demand-assignment equipment)

• If the number of stations, or the load on each one can fluctuate rapidly, some form of dynamic allocation of the frequency bands

is needed.

• Mechanism 794 simplex (64-kbps) PCM voice channels, : PCM channels were used in pairs to provide full dulplex service. : Total transponder bandwidth used was 50 Mbps for the uplink portion and another 50 Mbps for the downlink.

128-kbps common signaling channel. : Common signaling channel was divided into units of 50 msec. : A unit contained 50 slots of 1 msec (128 bits) : Each slot was “owned” by one of (not more than) 50 ground stations.

FDM(Frequency Division Multiplex) Continued

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It requires time synchronization for the slots, but this can be provided by a reference station, as described for slotted ALOHA above.

Slot assignment can be done in a centralized or a decentralized way.

Example of centralized slot assignment.

TDM(Time Division Multiplex)

• ACTS(Advanced Communication Technology Satellite)

Construction - Four independent 110-Mbps TDM channels - Each channels is organized as a sequence of 1-msec frames, each frame containing 1728 time slots. - Each time slot has 64-bit payload, allowing each one to hold a 64-kbps voice channel. - MCS(Master Control Station) : Time slot management requires a through knowledge of station geography to minimize the number of wasted time slots.

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TDM(Time Division Multiplex)

Operation

Dynamic TDM slot allocation

Continued

i) Satellite receives a frame and stores it in a 1728 entry onboard RAM.ii) Onboard computer copies each input entry to the corresponding output entry(possibly for the other antenna).

iii) Output frame is transmitted on the downlink.

1) There are more slots than stations, so each station can be assigned a home slot. (Binder, 1975)

G A F B B C D B

Owner G A F E B C D

G A B C D B

G A A E B B

G A F E B B B

Group1

Group2

Group3

Group4 collision

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TDM(Time Division Multiplex) Continued

2) Scheme is applicable even when the number of stations is unknown and varying. (Crowther et al., 1973) : Slots do not have permanent owners. Instead, stations compete for slots using slotted ALOHA. : Whenever a transmission is successful, the station making the successful transmission is entitled to that slot in the next frame as well. : As long as a station has data to send, it can continue doing so indefinitely.

A F C G B C D E

A B D E

A A A G

A A B D

Group1

Group2

Group3

Group4 collision

G

D

D

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TDM(Time Division Multiplex)

3) Requires stations to make advance requests before transmitting. (Roberts, 1973) : Each frame contains one special slot which is divided into V smaller subslots used to make reservations.

: When a station wants to send data, it broadcasts a short request frame in a randomly-chosen reservation subslot.

: If the reservation is successful, then the next regular slot is reserved.

Continued

Group1

Group2

Group3

Group4

Reservation subslots

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TDM(Time Division Multiplex)

Disadvantages

Continued

• TDM requires all stations to synchronize in time.

: It is not entirely trivial in practice because satellites tend to drift in orbit, which changes the propagation time to each ground station.

• TDM requires each ground station to be capable of extremely high burst speeds.

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CDMA(Code Division Multiple Access)

CDMA avoids the time synchronization problem and also the channel allocation problem.

Disadvantages

• The capacity of a CDMA channel in the presence of noise and uncoordinated stations is typically lower than what TDM can achieve.

• 128 chips /bit (a common value), although the bit rate may not be high, the chip rate will be, necessitating a fast transmitter.

• Few practicing engineers actually understand CDMA, which generally does not increase the chances of their using it, even if it is the best method for a particular application.